I. Field of The Invention
This invention relates generally to a method for producing high surface area amorphous transition metal chalcogenides. More specifically, the invention relates to a method for preparing high surface area amorphous molybdenum sulfides and selenides by a covalent exchange mechanism which produces novel compositions having advantageous properties for use in high energy density batteries or voltaic cells. The high surface area amorphous molybdenum chalcogenides of the invention are suitable for use as cathodes in battery cells because of their high activity, low weight, and rechargeable characteristics. They may also be used as catalysts, e.g., for hydrodesulfurization reactions.
II. Description of The Prior Art
The art has devoted substantial attention in recent years to the development of primary and secondary batteries for use in a variety of industrial, consumer and space age systems. Among the systems in development and which show substantial promise are those which comprise a light weight metal, e.g., an alkali metal, as anode, a nonaqueous, fused or solid electrolyte, and a transition metal chalcogenide as a cathode. Among the art describing such systems are U.S. Pat. Nos. 3,864,167, 3,791,867, and 3,988,164. The uses for such battery systems include electric vehicle propulsion, utility load leveling, and the standard automotive uses, i.e., engine starting, lighting, and ignition. Because of their high energy density, secondary lithium batteries comprising molybdenum chalcogenide as cathode are also of interest for space and military applications.
A number of transition metal sulfides and selenides (Group IV, V, VI and VII), and to a lesser extent oxides and tellurides, form a layered crystal structure reminiscent of that of graphite. Like graphite, these compounds are able to reversibly accommodate small molecules or ions between the layers, a process called intercalation. When charged ions are inserted and removed, the chalcogenide takes on a reversible net charge. So long as the intercalate fits without strain between the layers of the chalcogenide, this "reaction" occurs without physical change. It is this property that makes these materials useful as electrodes which must undergo many charge and discharge cycles without loss of physical integrity, as might be caused, for instance, by a change in crystal size or shape.
Lithium ions (Li.sup.+) are attractive charge carriers for a number of fundamental reasons. This leads naturally to the consideration of chalcogenides as cathodes in battery systems with lithium anodes and wherein charge transport is by ionic conduction. Many chalcogenides possess fair electronic conductivity and can thus transmit the charge delivered by ionic transport to a current collector via electron flow.
The theoretical maximum capacity of the chalcogenide for Li.sup.+ can be calculated from theory. The approach to that capacity and the speed with which it can be attained are functions of the physical state of the chalcogenide. Large planes, for instance, lead to inordinately long transport distances and times for the Li ions which can theoretically be accepted in the interior. Generally, very small particles are desirable. These present the maximum "edge" and minimum "interior" space. The art has recognized the value of transition metal chalcogenides, particularly molybdenum chalcogenides, as materials suited to the production of electrodes.
Pertinent U.S. patents illustrating the use of molybdenum chalcogenides in batteries are Dines et al., U.S. Pat. No. 4,323,480, Rao et al., U.S. Pat. No. 4,322,317, Sasu et al., U.S. Pat. No. 4,301,221, Dines et al., U.S. Pat. No. 4,299,892, Thompson et al., U.S. Pat. No. 4,237,204, Whittingham et al., U.S. Pat. No. 4,233,375, Haering et al., U.S. Pat. No. 4,224,390, Chianelli et al., U.S. Pat. No. 4,166,160, Pohl et al., U.S. Pat. No. 3,907,600, and Broadhead et al., U.S. Pat. No. 3,864,167.
The art has failed, however, to provide a satisfactory high activity, low weight, reversible, i.e., rechargeable, cathodic material which can be used in high current density batteries and which can be recharged at a rapid rate. For example, the titanium sulfide materials frequently used as cathodes have the disadvantage in that they have relatively low capacity as measured in watt-hours per unit of cathode material. It is believed that the titanium sulfide is limited by the ability of lithium ions in the electrolyte to intercalate into the cathode. Thus the cathode material after use becomes Li.sub.x TiS.sub.2 and x is believed to be less than 1 and as a practical matter only about 0.8 at a maximum. It is desirable that the value of x be greater than 1 and preferably as high as 4.
Molybdenum sulfide (MoS.sub.3) is an alternative to titanium disulfide. When MoS.sub.3 is used as a cathode in a lithium cell, Li.sub.x MoS.sub.3 is produced and x equals approximately 4. The difficulty with molybdenum sulfide as produced by prior art processes is that the rechargeability characteristic is unsatisfactory.
While molybdenum sulfide has received substantial attention from prior art workers, it is believed that the problem with its performance can be traced to its method of preparation. There are two known routes for the preparation of molybdenum sulfide: preparation via a thermal decomposition reaction, and preparation via a chemical decomposition reaction.
Thermal decomposition reactions typically take place at 200.degree. C. or higher temperatures in the presence of ammonia-containing compounds. The high vapor pressure of ammonia over the reaction causes difficulties. Thermal decomposition methods for the production of molydenum sulfide involve the decomposition of ammonium thiomolybdate.
Chemical decomposition reactions are unsatisfactory because any oxygen impurities present react with the molybdenum sulfide or other chalcogenide to form an insulating oxide layer on the surface of that material. Such oxide layers are detrimental to the use of the material as a cathode. Such surface imperfections may be seen by surface inspection. Chemical decomposition reactions lock water into the structure and that has a similar insulating and deleterious effect with respect to cathode usage.
Typical of the methods for producing molybdenum sulfide via chemical decomposition reactions are the methods taught in Dines et al., U.S. Pat. No. 4,299,892. A disadvantage of this prior art method is that the molybdenum sulfide obtained is always molybdenum disulfide (MoS.sub.2) and the compound is always obtained via an ionic-type precipitation. A further disadvantage of chemical decomposition reactions is that there is inevitable formation of an extra salt, such as, for example, lithium chloride (LiCl.sub.4). Such byproduct salts must be removed by a washing process usually in an aqueous system and this introduces water and oxygen into the chalcogenide with resultant surface contamination and deleterious effect on performance as a cathode. The ionic precipitations are also time consuming, requiring as much as twelve to twenty-four hours to complete even at elevated temperatures. In addition, the ionic exchange described by Dines et al. failed to produce chalcogenides of different stoichiometry but rather produced only the dichalcogenides.